Electrochemical reduction of carbon dioxide

ABSTRACT

Disclosed herein is a method for selectively reducing, using electrical energy, CO 2  to carbon monoxide or formic acid, a catalyst for use in the method, and an electrochemical reduction system. The method for producing carbon monoxide or formic acid by electrochemically reducing carbon dioxide of the present invention includes (a) reacting carbon dioxide with a metal complex represented by formula (1), and (b) applying a voltage to a reaction product of the carbon dioxide and the metal complex represented by formula (1):

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of prior U.S. applicationSer. No. 15/553,739, filed Aug. 25, 2017, the disclosure of which isincorporated herein by reference in its entirety. U.S. application Ser.No. 15/553,739 is the National Stage of PCT/JP2016/053558, filed Feb. 5,2016, the disclosure of which is incorporated herein by reference in itsentirety. U.S. application Ser. No. 15/553,739 claims priority toJapanese Application No. 2015-037839, filed Feb. 27, 2015, and JapaneseApplication No. 2015-160768, filed Aug. 18, 2015, the disclosures ofwhich are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to a method for electrochemically reducingcarbon dioxide to carbon monoxide or formic acid, and relates to acatalyst used therefor.

BACKGROUND ART

Currently, people are facing serious problems of global warming andexhaustion. of carbon resources. As means for solving these problems, acatalyst for converting light energy into chemical energy is attractingattention. It is expected that these problems should be solved all atonce carbon dioxide (CO₂) could be converted into a useful compoundusing inexhaustible solar energy. CO₂ is, however, an end product ofoxidation of carbon compounds, and hence is both physically andchemically very stable and has very low reactivity.

Recently, some techniques for converting CO₂ into a useful compoundthrough reduction have been reported. For example, Patent Literature 1describes a method for obtaining formic acid by reacting CO₂ andhydrogen in the presence of a catalyst, and Patent Literature 2describes a method for obtaining formic acid through reduction of CO₂caused by transferring, to a catalyst, an excited electron generatedthrough light irradiation of a semiconductor electrode. Besides, PatentLiterature 3 and Non Patent Literature 1 have reported a method forreducing CO₂ to carbon monoxide by bringing a rhenium complex intocontact with CO₂ and it the resultant with light. Furthermore, attemptshave been made to electrochemically reducing CO₂ in the presence of ametal complex catalyst (Patent Literature 4).

CITATION LIST Patent Literature

[Patent Literature 1] JP-A-2004-217632

[Patent Literature 2] JP-A-2011-82144

[Patent Literature 3] JP-A-2013-180943

[Patent Literature 4] JP-A-2013-193056

Non Patent Literature

[Non Patent Literature 1] J. Am. Chem, Soc. 2013, 135, 16825-16828

SUMMARY OF INVENTION Technical Problem

The methods described in Patent Literatures 1 and 2 require, however,hydrogen or a semiconductor and light irradiation for the reduction, andin addition, hydrogen is necessary for the reduction. in PatentLiterature 1, and thus these methods cannot be said energeticallyadvantageous. Besides, in Patent Literature 3 and Non Patent Literature1, another catalyst such as a ruthenium complex is necessary for aphotocatalytic reaction in addition to a reduction catalyst.Furthermore, in Patent Literature 4, a product resulting from theelectrochemical treatment of CO₂ is unknown.

On the other hand, if carbon monoxide (CO) or formic acid can beselectively obtained through the reduction of CO₂, the thus obtainedcarbon monoxide can be a material of extremely various hydrocarbons. Ahydrocarbon is a chemical energy material similarly to petroleum.Besides, formic acid can be used for easily producing hydrogen through areaction with a catalyst, and hence is expected as a liquid fuel whichstores hydrogen.

Accordingly, an object of the present invention is to provide a methodfor reducing CO₂ selectively to carbon monoxide or formic acid by usingelectrical energy, a catalyst for use in the method, and anelectrochemical reduction system.

Solution to Problem

Therefore, the present inventor made various examinations forelectrochemically performing reduction of CO₂ to carbon monoxide orformic acid, and found that CO₂ can be selectively and easily reduced tocarbon monoxide or formic acid by reacting CO₂ with a metal complexrepresented by formula (1) or formula (2) and applying a voltage to theresultant reaction product, and that this reduction reaction proceedseven if the concentration of CO₂ to be introduced is low, resulting inaccomplishing the present invention.

Specifically, the present invention provides the following [1] to [30]:

[1] A method for producing carbon monoxide by electrochemically reducingcarbon dioxide, comprising the following steps (a) and (b):

(a) reacting carbon dioxide with a metal complex represented by formula(1):

wherein

X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent,

R¹ represents a hydrocarbon group optionally having a substituent,

one, two or three of R², R³ and R⁴ are identical or different andrepresent a hydrocarbon group optionally having a substituent, the restrepresenting a hydrogen atom, and

one, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group; and

(b) applying a voltage to a reaction product of the carbon dioxide andthe metal complex represented by formula (1).

[2] The production method according to [1], wherein the aforementionedsteps (a) and (b) are performed within an electrochemical cell includinga working electrode and a counter electrode, and the method comprisesthe following steps (a1) and (31):

(a1) introducing carbon dioxide into a solution containing the metalcomplex held in the electrochemical cell; and

(b1) applying a negative voltage and a positive voltage respectively tothe working electrode and the counter electrode of the electrochemicalcell.

[3] The production method according to [2], wherein the carbon dioxideis introduced by introducing a carbon dioxide-containing gas into thesolution containing the metal complex.

[4] The production method according to any one of [1] to [3], whereinthe carbon dioxide to be reacted is a gas containing 0.03 to 100% ofcarbon dioxide.

[5] The production method according to any one of [1] to [4], whereinthe nitrogen atom-containing heterocycle including ring A and ring B isa heterocycle having a 2,2′-bipyridine structure optionally having asubstituent.

[6] The production method according to any one of [1] to [5], whereineach hydrocarbon group optionally having a substituent represented byR¹, R², R³, R⁴, X¹, X² and X³ is one selected from the group consistingof an alkyl group, an alkenyl group, a cycloalkyl group, a cycloalkenylgroup and an aromatic hydrocarbon group, each of which optionally hasone to three substituents selected from the group consisting of aprimary, secondary or tertiary amino group, a hydroxy group, an alkoxygroup, an aryloxy group, a halogen atom, a nitro group, a cyano group, aformyl group, an alkanoyl group and an arylcarbonyl group.

[7] A method for producing carbon monoxide from carbon dioxide, whereinthe carbon monoxide obtained by the production method according to anyone of [1] to [6] is used as a reducing agent.

[8] A method for producing a hydrocarbon-based compound, wherein thecarbon monoxide obtained by the production method according to any oneof [1] to [6] is used as a raw material.

[9] A catalyst for electrochemically reducing carbon dioxide to carbonmonoxide, represented by formula (1):

wherein

X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent,

R¹ represents a hydrocarbon group optionally having a substituent,

one, two or three of R², R³ and R⁴ are identical or different andrepresent a hydrocarbon group optionally having a substituent, the rest.representing a hydrogen atom, and

one, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group.

[10] The catalyst according to [9], wherein the nitrogen atom-containingheterocycle including ring A and ring B is a heterocycle having a2,2′-bipyridine structure optionally having a substituent.

[11] The catalyst according to [9] or [10], wherein each hydrocarbongroup optionally having a substituent represented by R¹, R², R³, R⁴, X¹,X² and X³ is one selected from the group consisting of an alkyl group,an alkenyl group, a cycloalkyl group, a cycloalkenyl group and anaromatic hydrocarbon group, each of which optionally has one to threesubstituents selected from the group consisting of a primary, secondaryor tertiary amino group, a hydroxy group, an alkoxy group, an aryloxygroup, a halogen atom, a nitro group, a cyano group, a formyl group, analkanoyl group and an. arylcarbonyl group.

[12] A method for producing formic acid by electrochemically reducingcarbon dioxide, comprising the following steps (a) and. (b):

(a) reacting carbon dioxide with a metal complex represented by formula(2):

wherein

M₁ represents manganese, ruthenium or iron,

X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent,

R¹ represents a hydrocarbon group optionally having a substituent,

one, two or three of R², R³ and R⁴ are identical or different andrepresent a hydrocarbon group optionally having a substituent, the restrepresenting a hydrogen atom, and

one, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group; and

(b) applying a voltage to a reaction product of the carbon dioxide andthe metal complex represented by formula (2).

[13] The production method according to [12], wherein the aforementionedsteps (a) and (b) are performed within an electrochemical cell includinga working electrode and a counter electrode, and the method comprisesthe following steps (a1) and (b1):

(a1) introducing carbon dioxide into a solution containing the metalcomplex held in the electrochemical cell; and

(b1) applying a negative voltage and a positive voltage respectively tothe working electrode and the counter electrode of the electrochemicalcell.

[14] The production. method according to [13], wherein the carbondioxide is introduced by introducing a carbon dioxide-containing gasinto the solution containing the metal complex.

[15] The production method according to any one of [12] to [14], whereinthe carbon dioxide to be reacted is a gas containing 0.03 to 100% ofcarbon dioxide.

[16] The production. method according to any one of [12] to [15],wherein the nitrogen atom-containing heterocycle including ring A andring B is a heterocycle having a 2,2′-bipyridine structure optionallyhaving a substituent.

[17] The production method according to any one of [12] to [16], whereineach hydrocarbon group optionally having a substituent represented byR¹, R², R³, R⁴, X¹, X² and X³ is one selected from the group consistingof an alkyl group, an alkenyl group, a cycloalkyl group, a cycloalkenylgroup and an aromatic hydrocarbon group, each of which optionally hasone to three substituents selected from the group consisting of aprimary, secondary or tertiary amino group, a hydroxy group, an alkoxygroup, an aryloxy group, a halogen atom, a nitro group, a cyano group, aformyl group, an alkanoyl group and an arylcarbonyl group.

[18] A catalyst for electrochemically reducing carbon dioxide to formicacid, represented by formula (2):

wherein

M₁ represents manganese, ruthenium or iron,

X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent,

R¹ represents a hydrocarbon group optionally having a substituent,

one, two or three of R², R³ and R⁴ are identical or different andrepresent a hydrocarbon group optionally having a substituent, the restrepresenting a hydrogen atom, and

one, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group.

[19] The catalyst accord in to [18], wherein the nitrogenatom-containing heterocycle including ring A and ring B is a heterocyclehaving a 2,2′-bipyridine structure optionally having a substituent.

[20] The catalyst according to [18] or [19], wherein each hydrocarbongroup optionally having a substituent represented by R¹, R², R³, R⁴, X¹,X² and X³ is one selected from the group consisting of an alkyl group,an alkenyl group, a cycloalkyl group, a cycloalkenyl group and anaromatic hydrocarbon group, each of which optionally has one to threesubstituents selected from the group consisting of a primary, secondaryor tertiary amino group, a hydroxy group, an alkoxy group, an aryloxygroup, a halogen atom, a nitro group, a cyano group, a formyl group, analkanoyl group and an arylcarbonyl group.

[21] A metal complex represented by formula (2a):

wherein

M₁ represents manganese, ruthenium or iron,

X represents O(CH₂)_(n)NR⁵R⁶, NR⁵R⁶ or PX¹X²X³,

Y represents CO, C(CH₂)_(n)NR⁵R⁶, NR⁵R⁶ or PX¹X²X³,

ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent,

one, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group,

R⁵ and R⁶ are identical or different and represent an alkyl group, ahydroxyalkyl group or a hydrogen atom, and

n represents a number of 2 to 8.

[22] The metal complex according to [21], wherein the nitrogenatom-containing heterocycle including ring A and ring B is a heterocyclehaving a 2,2′-bipyridine structure optionally having a substituent.

[23] The metal complex according to [21] or [22], wherein eachhydrocarbon group optionally having a substituent represented by X¹, X²and X³ is one selected from the group consisting of an alkyl group, analkenyl group, a cycloalkyl group, a cycloalkenyl group and an aromatichydrocarbon group, each of which optionally has one to threesubstituents selected from the group consisting of a primary, secondaryor tertiary amino group, a hydroxy group, an alkoxy group, an aryloxygroup, a halogen atom, a nitro group, a cyano group, a formyl group, analkanoyl group and an arylcarbonyl group.

[24] A carbon monoxide production system for producing carbon monoxideby electrochemically reducing carbon dioxide, the carbon monoxideproduction system comprising:

an electrochemical cell part equipped with a solution containing a metalcomplex, a working electrode and a counter electrode;

an injection part through which carbon dioxide is injected into thesolution containing the metal complex held in the electrochemical cellpart;

a voltage source capable of applying a positive or negative voltagebetween the working electrode and the counter electrode of theelectrochemical cell part; and

a discharge part discharging carbon monoxide generated within thesolution containing the metal complex, wherein the carbon monoxide isgenerated by applying a positive or negative voltage to a reactionproduct of the metal complex generated by the solution containing themetal complex and the carbon dioxide.

[25] The carbon monoxide production system according to [24],

wherein the metal complex is represented. by formula (1):

wherein

X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,

ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent,

R¹ represents a hydrocarbon group optionally having a substituent,

one, two or three of R², R³ and R⁴ are identical or different andrepresent a hydrocarbon group optionally having a substituent, the restrepresenting a hydrogen atom, and

one, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group.

[26] The carbon monoxide production system according to [24] or [25],wherein the carbon dioxide is fed without concentration in a feed partfeeding the carbon dioxide.

[27] The carbon monoxide production system according to any one of [24]to [26], further comprising a carbon monoxide detection part detecting aconcentration of the carbon monoxide discharged from the solutioncontaining the metal complex.

[28] The carbon monoxide production system according to [27], whereinthe carbon monoxide detection part is a gas chromatography.

[29] The carbon monoxide production system according to any one of [24]to [28], wherein the nitrogen atom-containing heterocycle including ringA and ring B is a heterocycle having a 2,2′-bipyridine structureoptionally having a substituent.

[30] The carbon monoxide production system according to any one of [24]to [29], wherein each hydrocarbon group optionally having a substituentrepresented by R¹, R², R³, R⁴, X¹, X² and X³ is one selected from thegroup consisting of an alkyl group, an alkenyl group, a cycloalkylgroup, a cycloalkenyl group and an aromatic hydrocarbon group, each ofwhich optionally has one to three substituents selected from the groupconsisting of a primary, secondary or tertiary amino group, a hydroxygroup, an alkoxy group, an aryloxy group, a halogen atom, a nitro group,a cyano group, a formyl group, an alkanoyl group and an aryl carbonylgroup.

Effects of Invention

If a catalyst of the present invention and an electrochemical treatmentare employed, carbon monoxide (CO) or formic acid can be efficientlyproduced from CO₂ by simple means even if the CO₂ is at a lowconcentration. Accordingly, carbon monoxide or formic acid which can bevarious chemical materials can be efficiently produced from aCO₂-containing waste gas of facilities, such as a thermal power stationor an ironworks, in which a combustion waste gas of an organic matterincluding petroleum is generated. Accordingly, carbon monoxide or formicacid which can be a raw material of a useful and energy-storing chemicalsubstance such as hydrocarbon or hydrogen can be produced from acombustion waste gas of a fossil fuel such as petroleum, coal or naturalgas, and therefore, contribution can be made to both energy reuse andCO₂ emission reduction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram of a carbon monoxide production system ofthe present invention.

FIG. 2 is a conceptual diagram of an example of a reaction-side cellchamber of the carbon monoxide production system of the present.invention. Like reference signs are used in this drawing to refer tolike elements of FIG. 1.

FIG. 3 illustrates IR spectrum change (solvent: DMF-TEOA (5:1 v/v))obtained by allowing CO₂ to pass for 30 minutes through a solution 4hours after adding TEOA. Spectrum obtained before the passage: grayline, spectrum obtained after the passage: black line

FIG. 4 illustrates IR spectrum change (solvent: DMF-TEGA (5:1 v/v))obtained by allowing Ar to pass through the solution after allowing theCO₂ to pass (after 0 to 120 minutes).

FIG. 5 illustrates an ESI-MS spectrum (solvent: MeCN) of a DMF-TEOAmixed solution (5:1 v/v) containing Re—CO₂-TEOA.

FIG. 6 illustrates IR spectra (black solid lines) obtained after passageof the ambient air and curve fitting curves. (Peaks corresponding to acomplex in which DMF is coordinated, a complex in which —O—CO—OCH₂CH₂NR₂(R═CH₂CH₂OH) is coordinated, and a complex in which TEOA is coordinated,arranged in this order from a right peak illustrated with a dotted line.A gray solid line is formed by adding up these three peaks.)Substituents in 4,4′-position: (a) hydrogen, (b) a methyl group, (c) amethoxy group, and (d) a bromo group

FIG. 7 illustrates amount of CO generated and current change obtained inelectrochemical CO₂ reduction experiment using Re—CO₂-TEOA as acatalyst.

FIG. 8 illustrates amount of CO generated and current change obtained byadding TEOA.

FIG. 9 illustrates measurement results of cyclic voltammetry (CV) of aMn complex performed for setting an application voltage. In thisdrawing, “baseline” corresponds to a measurement result obtained withoutthe complex under CO₂ atmosphere, “under Ar” corresponds to ameasurement result obtained with or without the complex under Aratmosphere, and “under CO₂” corresponds to a measurement result obtainedwith or without the complex under CO₂ atmosphere.

FIG. 10 illustrates measurement results of the cyclic voltammetry (CV)of a Mn complex performed for setting an application voltage. A dottedline corresponds to a result obtained under Ar atmosphere. A solid. linecorresponds to a result obtained. under CO₂ atmosphere.

FIG. 11 illustrates current value change obtained in an electrochemicalCO₂ reduction experiment using Mn—CO₂-TEOA as a catalyst.

FIG. 12 illustrates amount of CO generated obtained in theelectrochemical CO₂ reduction experiment using Mn—CO₂-TEOA as acatalyst.

FIG. 13 illustrates IR spectra of various manganese triethanolamineadduct complexes with a CO₂ concentration of 10%.

FIG. 14 illustrates IR spectrum change obtained in DMF-DEOA of a Mncomplex under atmosphere of various CO₂ concentrations.

FIG. 15 illustrates measurement results of the cyclic voltammetry (CV)of a Mn complex in DMF-DEOA under CO₂atmosphere performed for setting anapplication voltage.

FIG. 16 illustrates amount of formic acid generated and current valuechange obtained in an electrochemical CO₂ reduction experiment usingMn—CO₂-DEOA as a catalyst.

Description of Embodiment

A catalyst used in electrochemical reduction of CO₂ to CO of the presentinvention is a metal complex represented by formula (1):

wherein

X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³;

Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³;

ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent;

R¹ represents a hydrocarbon group optionally having a substituent;

one, two or three of R², R³ and R⁴ are identical or different andrepresent a hydrocarbon group optionally having a substituent, the restrepresenting a hydrogen atom; and

one, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group.

On the other hand, a catalyst used in electrochemical reduction of CO₂to formic acid of the present invention is a metal complex representedby formula (2):

wherein

M₁ represents manganese, ruthenium or iron;

X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³;

Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³;

ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent;

R¹ represents a hydrocarbon group optionally having a substituent;

one, two or three of R², R³ and R⁴ are identical or different andrepresent a hydrocarbon group optionally having a substituent, the restrepresenting a hydrogen atom; and

one, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group.

In formula (2), M₁ is preferably manganese or ruthenium, and morepreferably manganese.

In formulas (1) and (2) , X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³, andY represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³. X and Y may be identicalor different. Here, R¹ represents a hydrocarbon group optionally havinga substituent. One, two or three of R², R³ and R⁴ are identical ordifferent and represent a hydrocarbon group optionally having asubstituent, the rest representing a hydrogen atom.

One, two or three of X¹, X² and X³ are identical or different andrepresent a hydrocarbon group optionally having a substituent or ahydrocarbon oxy group optionally having a substituent, the restrepresenting a hydrogen atom or a hydroxy group.

The hydrocarbon groups optionally having a substituent represented byR¹, R², R³, R⁴, X¹, X² and X³ are identical or different, and arepreferably any one of an alkyl group, an alkenyl group, a cycloalkylgroup, a cycloalkenyl group and an aromatic hydrocarbon group, each ofwhich optionally has one to three substituents selected from the groupconsisting of a primary, secondary or tertiary amino group, a hydroxygroup, an alkoxy group, an aryloxy group, a halogen atom, a nitro group,a cyano group, a formyl group, an alkanoyl group and an arylcarbonylgroup.

The alkyl group can be a straight or branched chain alkyl group having 1to 20 carbon atoms, is preferably a straight or branched chain alkylgroup having 1 to 12 carbon atoms, and is more preferably a straight orbranched chain alkyl group having 1 to 6 carbon atoms. Specific examplesinclude a methyl group, an ethyl group, a n-propyl group, an isopropylgroup, a n-butyl group, an isobutyl group, a sec-butyl group, atest-butyl group, a n-pentyl group and a n-hexyl group.

The alkenyl group can be a straight or branched chain alkenyl grouphaving 2 to 20 carbon atoms, is preferably a straight or branched chainalkenyl group having 2 to 12 carbon atoms, and is more preferably astraight or branched chain alkenyl group having 2 to 6 carbon atoms.Specific examples include a vinyl group, a 2-propenyl group, a1-propenyl group and a 1-butenyl group.

The cycloalkyl group can be a C₃-C₈ cycloalkyl group, and specificexamples include a cyclopropyl group, a cyclobutyl group, a cyclopentylgroup and a cyclohexyl group. The cycloalkenyl group can be a C₃-C₈cycloalkenyl group, and specific examples include a cyclobutenyl group,a cyclopentenyl group and a cyclohexenyl group.

The aromatic hydrocarbon group can be a C₆-C₁₄ aromatic hydrocarbongroup, and specific examples include a phenyl group, a naphthyl groupand a phenanthrenyl group.

The hydrocarbon oxy groups optionally having a substituent representedby X¹, X² and X³ are identical or different, and can be any one of analkoxy group, an alkenyloxy group, a cycloalkyloxy group, acycloalkenyloxy group and an aromatic hydrocarbon oxy group, each ofwhich optionally has one to three substituents selected from the groupconsisting of a primary, secondary or tertiary amino group, a hydroxygroup, an alkoxy group, an aryloxy group, a halogen atom, a nitro group,a cyano group, a formyl group, an alkanoyl group, and an arylcarbonylgroup.

The alkoxy group can be a straight or branched chain alkoxy group having1 to 20 carbon atoms, is preferably a straight or branched chain alkoxygroup having 1 to 12 carbon atoms, and is more preferably a straight orbranched chain alkoxy group having 1 to 6 carbon atoms. Specificexamples include a methoxy group, an ethoxy group, a n-propyloxy group,an isopropyloxy group, a n-butyloxy group, an isobutyloxy group, asec-butyloxy group, a test-butyloxy group, a n-pentyloxy group, and an-hexyloxy group.

The alkenyloxy group can be a straight or branched chain alkenyloxygroup having 2 to 20 carbon atoms, is preferably a straight or branchedchain alkenyloxy group having 2 to 12 carbon atoms, and is morepreferably a straight or branched chain alkenyloxy group having 2 to 6carbon atoms. Specific examples include a vinyloxy group, a2-propenyloxy group, a 1-propenyloxy group, a 1-butenyloxy group, andthe like.

The cycloalkyloxy group can be a C₃-C₈ cycloalkyloxy group, and specificexamples include a cyclopropyloxy group, a cyclobutyloxy group, acyclopentyloxy group and a cyclohexyloxy group. The cycloalkenyloxygroup can be a C₃-C₈ cycloalkenyloxy group, and specific examplesinclude a cyclobutenyloxy group, a cyclopentenyloxy group, acylohexenyloxy group, and the like.

The aryloxy group can be a C₆-C₁₄ aryloxy group, and specific examplesinclude a phenyloxy group, a naphthyloxy group, a phenanthrenyloxygroup, and the like.

As the group which can be substituted for such a hydrocarbon group orhydrocarbon oxy group, one to three groups selected from the groupconsisting of an amino group, a C₁₋₆ alkylamino group, a di(C₁₋₆ alkyl)amino group, a di(hydroxy C₁₋₆ alkyl)amino group, a hydroxy C₁₋₆alkylamio group, a hydroxy group, a C₁₋₆ alkoxy group, a C₁₋₁₄ aryloxygroup, a halogen atom, a nitro group, a cyano group, a formyl group, aC₁₋₆ alkanoyl group and a C₆₋₁₄ arylcarbonyl group are more preferred.Besides, one to three groups selected from the group consisting of anamino group, a C₁₋₆ alkylamino group, a di(C₁₋₆ alkyl)amino group, ahydroxy C₁₋₆ alkylamino group, a di(hydroxy C₁₋₆ alkyl)amino group, ahydroxy group, a C₁₋₆ alkoxy group, a C₁₋₁₄ aryloxy group, and a halogenatom are further preferred.

One, two or three of R², R³ and R⁴ represent any of the above-describedhydrocarbon groups, and the rest represents a hydrogen atom. Besides,one, two or three of X¹, X² and X³ represent any of the above-describedhydrocarbon groups or hydrocarbon oxy groups, and the rest represents ahydrogen atom or a hydroxy group.

More preferable X is OR¹ or NR¹R²R³.

Further preferable X is —OC₂₋₈ alkyl NHC₂₋₈ alkyl OH, —OC₂₋₈ alkylN(C₂₋₈ alkyl OH)₂, —NH(C₂₋₈ alkyl OH) or —N(C₂₋₈ alkyl OH)₂. Furtherpreferable X is —OC₂₋₆ alkyl NHC₂₋₆ alkyl OH, —OC₂₋₆ alkyl N (C₂₋₆ alkylOH)₂, —NH(C₂₋₆ alkyl OH), or —N(C₂₋₆ alkyl OH)₂. Still furtherpreferable X is —OC₂H₄NHC₂H₄OH, —C₂H₄N(C₂H₄OH)₂, —NH(C₂H₄OH) or—N(C₂H₄OH)₂. Besides, more preferable Y is CO, OR¹ or NR¹R²R³, andfurther preferable Y is CO.

As the nitrogen atom-containing heterocycles including ring A and ringB, a heterocycle having a 2,2′-bipyridine structure optionally having asubstituent is preferred. As a group which can be substituted in theheterocycle, one to four groups selected from the group consisting of analkyl group, an alkoxy group, an aryloxy group, a halogen atom and analkanoyl group are preferred, and one to four groups selected from thegroup consisting of a C₁₋₆ alkyl group, a C₁₋₆ alkoxy group, a C₆₋₁₄aryloxy group, a halogen atom and a C₁₋₆ alkanoyl group are morepreferred.

As the heterocycle having the 2,2′-bipyridine structure, for example, aheterocycle represented by the following formula (3) or (4) ispreferred:

In the formulas, R⁷, R⁸, R⁹, R¹⁰ and R¹¹ are identical or different, andrepresent a hydrogen atom, an alkyl group, an alkoxy group, an aryloxygroup, a halogen atom or an alkanoyl group.

Among these, the heterocycle of formula (3) is more preferred. Morespecifically, 2,2′-pyridine, 4,4′-dimethyl-2,2′-bipyridine,4,4′-dibromo-2,2′-bipyridine and 4,4′-dimethoxy-2,2′-bipyridine arepreferred.

Among metal complexes represented by the aforementioned formula (2), ametal complex represented by the following formula (2a) is novel, and ismore preferred:

In the formula, M₁ represents manganese, ruthenium or iron; X representsO(CH₂)_(n)NR⁵R⁶, NR⁵R⁶ or PX¹X²X³; Y represents CO, O(CH₂)_(n)NR⁵R⁶,NR⁵R⁶ or PX¹X²X³; ring A and ring B are identical or different andrepresent a nitrogen atom-containing heterocycle optionally having asubstituent; one, two or three of X¹, X² and X³ are identical ordifferent and represent a hydrocarbon group optionally having asubstituent or a hydrocarbon oxy group optionally having a substituent,the rest representing a hydrogen atom or a hydroxy group; R⁵ and R⁶ areidentical or different and represent an alkyl group, a hydroxyalkylgroup or a hydrogen atom; and n represents a number of 2 to 8.

As M¹, manganese or ruthenium is more preferred, and manganese isfurther preferred.

As the nitrogen atom-containing heterocycles including ring A and ringB, a heterocycle having a 2,2′-bipyridine structure optionally having asubstituent is preferred, a heterocycle of the aforementioned formula(3) or (4) is more preferred, a heterocycle of formula (3) is furtherpreferred, and 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, and4,4′-dibromo-2,2′-bipyridine are particularly preferred.

R⁵ and R⁶ in O(CH₂)_(n)NR⁵R⁶ and NR⁵R⁶ are identical or different, andrepresent an alkyl group, a hydroxyalkyl group or a hydrogen atom. Morespecifically, R⁵ and R⁶ can be a C₁₋₆ alkyl group, a hydroxy C₁₋₆ alkylgroup or a hydrogen atom, and are preferably a C₁₋₄ alkyl group, ahydroxy C₁₋₆ alkyl group or a hydrogen atom.

As the hydrocarbon groups optionally having a substituent represented byX¹, X² and X³, any one of an alkyl group, an alkenyl group, a cycloalkylgroup, a cycloalkenyl group and. an aromatic hydrocarbon group, each ofwhich optionally has one to three substituents selected from the groupconsisting of a primary, secondary or tertiary amino group, a hydroxygroup, an alkoxy group, an aryloxy group, a halogen atom, a nitro group,a cyano group, a formyl group, an alkanoyl group and an arylcarbonylgroup, is preferred.

The hydrocarbon oxy groups optionally having a substituent representedby X¹, X² and X³ are identical or different, and can be any one of analkoxy group, an alkenyloxy group, a cycloalkyloxy group, acycloalkenyloxy group and an aryloxy group, each of which optionally hasone to three substituents selected from the group consisting of aprimary, secondary or tertiary amino group, a hydroxy group, an alkoxygroup, an aryloxy group, a halogen atom, a nitro group, a cyano group, aformyl group, an alkanoyl group and an arylcarbonyl group.

More preferable X is O(CH₂)_(n)NR⁵R⁶ or NR⁵R⁶.

Further preferable X is —OC₂₋₈ alkyl NHC₂₋₈ alkyl OH, —OC₂₋₈ alkylN(C₂₋₈ alkyl OH)₂, —NH(C₂₋₈ alkyl OH) or —N(C₂₋₈ alkyl OH)₂. Furtherpreferable X is —OC₂₋₆ alkyl NHC₁₋₆ alkyl OH, —OC₂₋₆ alkyl N (C₂₋₆ alkylOH)₂, —NH(C₂₋₆ alkyl OH) or —N(C₂₋₆ alkyl OH)₂. Further preferable X is—OC₂H₄NHC₂H₄OH, —OC₂H₄N(C₂H₄OH)₂, —NH(C₂H₄OH) or —N(C₂H₄OH)₂. Besides,more preferable Y is CO, O(CH₂)_(n)NR⁵R⁶ or NR⁵R⁶, and furtherpreferable Y is CO.

The metal complex represented by formula (1) or (2) can be produced inaccordance with, for example, the following reaction formulas:

In the formulas, M represents rhenium, manganese, ruthenium or iron,and. A, B, X and Y are the same as defined above.

Specifically, an acetonitrile (MeCN)-coordinated metal complex offormula (5) is converted. into a solvent-coordinated complex (6) througha reaction with a solvent having comparatively low coordination abilitysuch as dimethylformamide, and the resultant complex is reacted with X—Hand/or Y—H (7) in a basic condition, and thus, the metal complex offormula (1) or (2) can be produced. The conversion from theacetonitrile-coordinated complex (5) to the solvent-coordinated complexmay be performed by dissolving the complex of formula (5) in theabove-described solvent, and allowing the resultant to stand stillovernight in a dark place under Ar atmosphere. Next, for producing thecomplex of formula (1) or (2), the complex of formula (6) is added toX—H (7), and the resultant is allowed to stand still for several hoursin a dark place under Ar atmosphere.

A method for producing CO from CO₂ by electrochemical reduction of thepresent invention is characterized by including the following steps (a)and (b):

(a) a step of reacting carbon dioxide with a metal complex representedby formula (1) described above; and

(b) a step of applying a voltage to a reaction product of the carbondioxide and the metal complex represented by formula (1).

Reactions of the steps (a) and (b) are regarded to proceed in accordancewith the following reaction formulas:

In the formulas, A, B, X and Y are the same as defined above.

Specifically, through the reaction between the metal complex of formula(1) and CO₂, a CO₂ adduct represented by formula (8) is generated, andwhen a voltage is applied to this adduct, CO is released. It is notedthat the generation of the CO₂ adduct of formula (8) can be confirmedbased on an IR spectrum, an MS spectrum and an NMR spectrum.

The reaction may be performed in an electrolyte solution, namely, apolar solvent, and from the viewpoint that an oxygen atom produced as aby-product simultaneously with CO released from the CO₂ adduct offormula (8) is changed into water through protonation, a protic polarsolvent is preferably used. Examples of the protic polar solvent includewater, an alcohol-based solvent, an amine-based solvent, a thiol-basedsolvent and an amino alcohol-based solvent. Among these, a solventcorresponding to X and/or of formula (1) is particularly preferablyused.

The amount of the metal complex of formula (1) to be used is preferably0.01 mM to 100 mM in the electrolyte solution, and more preferably 0.05mM to 10 mM.

The CO₂ to be introduced. is not necessarily 100% CO₂, and the COgeneration reaction proceeds even if a gas containing 0.03% to 100% ofCO₂ is used. The concentration of 0.03% of CO₂ gas corresponds to theCO₂ concentration in the ambient air. Besides, CO₂ of a waste gascontaining about 10% of CO₂ from a thermal power station or the like canbe directly used without concentration.

Besides, the CO₂ can be easily introduced by introducing aCO₂-containing gas into the electrolyte solution, for example, bybubbling a CO₂-containing gas through the electrolyte solution.

Next, for setting an application voltage, it is significant to grasp anapplication voltage level by precedently performing cyclic voltammetry(CV) measurement. The cyclic voltammetry (CV) measurement is a method inwhich an electrode potential is linearly swept to measure a responsecurrent. In the present invention, the cyclic voltammetry measurement isperformed (a) in the absence (blank) or in the presence of the metalcomplex of the present invention in the electrolyte by introducing (b)Ar gas and (c) a CO₂-containing gas. When a current-potential curve isobtained in the condition (c), an application voltage (a reductionpotential) can be obtained on the basis of a rising potential of theresponse current. Incidentally, the voltage may be applied whileperforming the reaction within an electrochemical cell including aworking electrode and a counter electrode. The voltage is preferably 1.0V to 2.5 V vs. Ag/AgNO₃.

Specifically, the steps (a) and (b) are performed within anelectrochemical cell including a working electrode and a counterelectrode, and the following steps (a1) and (b1) are preferablyperformed:

(a1) a step of introducing carbon dioxide into a solution containing themetal complex held in the electrochemical cell; and

(b1) a step of applying a negative voltage and a positive voltagerespectively to the working electrode and the counter electrode of theelectrochemical cell.

More specifically, it is preferable to use a system, for example, asillustrated in FIG. 1, for producing carbon monoxide from carbon dioxidethrough electrochemical reduction, the system including anelectrochemical cell part equipped with a solution (1) containing ametal complex, a working electrode (4) and a counter electrode (6); aninjection part (an injection port) (2) through which carbon dioxide isinjected into the solution containing the metal complex held in theelectrochemical cell part (1); a potentiostat (8) including a voltagesource capable of applying a positive or negative voltage between theworking electrode (4) and the counter electrode (6) of theelectrochemical cell part; and a discharge part (a discharge port) (3)through which carbon monoxide generated within the solution containingthe metal complex is discharged, so that the carbon monoxide can begenerated by applying a positive or negative voltage to a reactionproduct of the metal complex generated by the solution containing themetal complex and the carbon dioxide.

FIG. 2 illustrates an apparatus more practical than that illustrated inFIG. 1. The description will now be given on the basis of FIGS. 1 and.2.

In FIGS. 1 and 2, a reference sign (2) denotes a CO₂ injection part(injection port), through which CO₂ contained in white dots illustratedin FIG. 2 is introduced into a solution containing a metal complex. InFIGS. 1 and 2, a reference sign (1) denotes an electrochemical cellpart, and the cell part includes a solution including a workingelectrode (4), and a reference electrode (5) and a counter electrode(6). Addition of CO₂ and a reduction reaction to CO are carried outthrough the metal complex of the working electrode. Glassy carbon or thelike is used as the working electrode. Platinum or the like is used asthe counter electrode.

In FIG. 1, a reference sign (8) denotes a potentiostat for applying apositive or negative voltage to the working electrode and the counterelectrode of the electrochemical cell part.

In FIG. 2, a reference sign (3) denotes a gas discharge part fordischarging CO (illustrated as gray dots) generated within the solutioncontaining the metal complex. This CO discharge part can be providedwith any of various CO sensors (of semiconductor type, thermalconductivity type and the like) or a detection part (a detector)detecting generation of carbon monoxide by gas chromatography(Micro-GC).

The system of the present invention can continuously produce CO from aCO₂-containing gas having a concentration of about 10% as illustratedin. FIGS. 1 and 2, and therefore, can be installed in facilities whereCO₂ is generated through combustion of organic substances, such as athermal power station, a cement manufacturing facility and a glassmanufacturing facility. Besides, it can be installed in a facility whereFe₂O₃ is reduced with CO, such as a blast furnace of an ironworks. Inthis case, the CO obtained by the method or the system of the presentinvention can be used as a reducing agent, so as to reproduce CO byusing generated CO₂ as a raw material. Besides, if CO obtained by themethod or the system of the present invention is used as a raw material,a wide range of hydrocarbon-based compounds can be produced.

A method for producing formic acid by electrochemically reducing CO₂ ofthe present invention is characterized by including the following steps(a) and (b):

(a) a step of reacting carbon dioxide with. a metal complex representedby formula (2) described above; and

(b) a step of applying a voltage to a reaction product of the carbondioxide and the metal complex represented by formula (2).

Reactions of the steps (a) and (b) are regarded to proceed in accordancewith the following reaction formulas:

In the formulas, A, B, M₁, X and Y are the same as defined above.

Specifically, through the reaction between the metal complex of formula(2) and CO₂, a CO₂ adduct represented by formula (9) is generated, andwhen a voltage is applied to this adduct, formic acid is released. It isnoted that the generation of the CO₂ adduct of formula (9) can beconfirmed based on an IR spectrum and an MS spectrum.

The reaction may be performed in an electrolyte solution, namely, apolar solvent, and from the viewpoint that formic acid is released fromthe CO₂ adduct of formula (9) , a protic polar solvent is preferablyused. Examples of the protic polar solvent include water, analcohol-based solvent, an amine-based solvent, a thiol-based solvent andan amino alcohol-based solvent. Among these, a solvent corresponding toX and/or Y of formula (2) is particularly preferably used.

The amount of the metal complex of formula to be used is preferably 0.01mM to 100 mM in the electrolyte solution, and more preferably 0.05 mM to10 mM.

The CO₂ to be introduced is not necessarily 100% CO₂, and the COgeneration reaction proceeds even if a gas containing 0.03% to 100% ofCO₂ is used. The concentration of 0.03% of CO₂ gas corresponds to theCO₂ concentration in the ambient air. Besides, CO₂ of a waste gascontaining about 10% of CO₂ from a thermal power station or the like canbe directly used without concentration.

Besides, the CO₂ can be easily introduced by introducing aCO₂-containing gas into the electrolyte solution, for example, bybubbling a CO₂-containing gas through the electrolyte solution.

Next, for setting an application voltage, it is significant to grasp anapplication voltage level by precedently performing the cyclicvoltammetry (CV) measurement. The cyclic voltammetry (CV) measurement isa method in which an electrode potential is linearly swept to measure aresponse current. In the present invention, the cyclic voltammetrymeasurement is performed (a) in the absence (blank) or in the presenceof the metal complex of the present invention in the electrolyte byintroducing (b) Ar gas and (c) a CO₂-containing gas. When acurrent-potential curve is obtained in the condition (c), an applicationvoltage (a reduction potential) can be obtained on the basis of a risingpotential of the response current. Incidentally, the voltage may beapplied while performing the reaction within an electrochemical cellincluding a working electrode and a counter electrode. The voltage ispreferably 1.0 V to 2.5 V vs. Ag/AgNO₃.

According to the method of the present invention, formic acid can becontinuously produced from a CO₂-containing gas having a concentrationof about 0.03%, and therefore, the method can be employed in facilitieswhere CO₂ is generated through combustion of organic substances, such asa thermal power station, a cement manufacturing facility and a glassmanufacturing facility.

EXAMPLES

Next, the present invention will be described in more detail withreference to examples.

Synthesis Example 1

Re(CO)₅Br

Four (4) mL of bromine was added in a dropwise manner to dichloromethane(8 mL) containing Re₂(CO)₁₀ (4.71 g, 7.21 mmol), and the resultant wasstirred at room temperature for 10 minutes. A white solid thus separatedwas filtered off.

Yield: 5.47 g (13.4 mmol), Yield: 93.1%

FT-IR in CH₂Cl₂ ν(CO)/cm⁻¹: 2154, 2046, 1988

Synthesis Example 2

Re(bpy)(CO)₃Br (sometimes abbreviated as Re—Br)

A toluene solution (60 ms) containing Re(CO)₆Br (3.00 g, 7.38 mmol) and2,2′-bipyridine(bpy) (1.27 g 8.14 mmol) was heated to reflux overnight.A yellow solid thus separated was filtered off and dried under reducedpressure. The resultant was purified by recrystallization usingacetonitrile/diethyl ether/hexane.

Yield: 3.63 g (7.18 mmol), Yield: 97.3%

ESI-MS in CH₂Cl₂ m/z=468 [M-PF₆ ⁻]⁺

FT-IR in CH₂Cl₂ ν(CO)/cm⁻¹: 2024, 1923, 1901

¹H NMR in CDCl₃: δ (ppm)=9.09 (d, J=7.0 Hz, 2H, bpy-6,6′), 8.21 (d,J=7.0 Hz, 2H, bpy-3,3′), 8.07 (dd, J=7.0, 7.0 Hz, 2H, bpy-4,4′), 7.55(dd, J=7.0, 7.0 Hz, 2H, bpy-5,5′)

Elemental analysis: Calcd. (%) for C₁₃H₁₀N₂O₃BrRe: C 30.84; H 1.59; N5.53

Found: C 30.86; H 1.46; N 5.61.

Synthesis Example 3

Re(dmb)(CO)₃Br

A toluene solution (60 mL) containing Re(CO)₅Br (1.10 g, 2.71 mmol) and4-4′-dimethyl-2,2′-bipyridine (dmb) (0.55 g, 3.00 mmol) was heated toreflux overnight. A yellow solid thus separated was filtered off anddried under reduced pressure.

Yield: 1.40 g (2.62 mmol), Yield: 96.7%

FT-IR in CH₂Cl₂ ν(CO)/cm⁻¹: 2022, 1920, 1898

Synthesis Example 4

Re{(MeO)₂bpy}(CO)₃Br

A toluene solution (60 mL) containing Re(CO)₅Br (499 mg, 1.23 mmol) and4,4′-dimethoxy-2,2′-bipyridine (1MPO)₂bpy (321 mg, 1.483 mmol) washeated to reflux overnight. A yellow solid thus separated was filteredoff and dried under reduced pressure.

Yield: 652 mg (1.15 mmol), Yield: 93.7%

FT-IR in CH₂Cl₂ ν(CO)/cm⁻¹: 2022, 1918, 1895

Synthesis Example 5

Re(Br₂bpy)(CO)₃Br

A toluene solution (60 mL) containing Re(CO)₅Br (503 mg, 1.24 mmol) and4-4′-dibromo-2,2′-bipyridine (Br₂bpy) (464 mg, 1.48 mmol) was heated toreflux overnight. A yellow solid thus separated was filtered off anddried under reduced pressure.

Yield: 803 mg (1.21 mmol), Yield: 97.7%

FT-IR in CH₂Cl₂ ν(CO)/cm⁻¹: 2026, 1928, 1905

Synthesis Example 6

[Re(bpy)(CO)₃(MeCN)](PF₆) (sometimes abbreviated as Re—MeCN)

Acetonitrile (60 mL) containing Re(bpy) (CO)₃Br (596 mg, 1.18 mmol) andAgPF₆ (327 mg, 1.29 mmol) was heated to reflux overnight. AgBr thusseparated was removed over a Celite layer, and the solvent was distilledoff under reduced pressure. To the resultant, saturated NH₄PF₆ inacetonitrile/water (1:1 v/v) was added, and the acetonitrile was slowlydistilled off under reduced pressure to obtain a pale yellow solid. Thissolid was recrystallized from acetonitrile/diethyl ether/hexane, and ayellow solid thus obtained was dried under reduced pressure.

Yield: 363 mg, (5.92×10⁻¹ mmol), Yield: 50.3%

ESI-MS in MeCN m/z=468 [M-PF₆ ⁻]⁺

FT-IR in MeCN ν(CO)/cm⁻¹: 2041, 1938

¹H NMR in CD₃CN (298 MHz): δ (ppm)=9.02 (dd, J=5.6, 1.5 Hz, 2H,bpy-6,6′), 8.47 (dd, J=8.2, 1.1 Hz, 2H, bpy-3,3′), 8.28 (ddd, J=8.2,8.2, 1.5 Hz, 2H, bpy-4,4′), 7.71 (ddd, J=6.2, 5.6, 1.1 Hz, 2H,bpy-5,5′), 2.03 (s, 3H, CH₃CN)

Elemental analysis: Calcd. (%) for C₁₅H₁₃N₃O₃RePF₆: C 29.40; H 1.81; N6.86

Found: C 29.35; H 1.65; N 6.91

Synthesis Example 7

[Re(dmb)(CO)₃(MeCN)](PF₆) (sometimes abbreviated as (Re(Me)MeCN)

An acetonitrile solution. (60 mL) containing Re(dmb) (CO)₃Br (500 mg,9.34×10⁻¹ mmol) and AgPF₆ (303 mg, 1.19 mmol) was heated to refluxovernight. AgBr thus separated was removed over a Celite layer, and thesolvent was distilled off under reduced pressure. To the resultant,saturated NH₄PF₆ in acetonitrile/water (1:1 v/v) was added, and theacetonitrile was slowly distilled off under reduced pressure to obtain apale yellow solid. This solid was recrystallized fromacetonitrile/diethyl ether/hexane, and a pale yellow sold thus obtainedwas dried under reduced pressure.

Yield: 478.6 mg, (7.47×10⁻¹ mmol), Yield: 79.9%

ESI-MS in MeCN m/z=496 [M-PF₆ ⁻]⁺

FT-IR in MeCN ν(CO)/cm⁻¹: 2039, 1935 ¹H-NMR in CD₃CN (298 MHz): δ(ppm)=8.82 (d, J=5.5 Hz, 2H, bpy-6,6′), 8.32 (s, 2H, bpy-3,3′), 7.52 (d,J=5.5 Hz, 2H, bpy-5,5′), 2.58 (s, 6H, bpy-CH₃), 2.04 (s, 3H, CH₃—CN)

Elemental analysis: Calcd. (%) for C₁₇H₁₅N₃O₃RePF₆: C 31.88; H 2.36; N6.56

Found: C 31.85; H 2.19; N 6.59

Synthesis Example 8

[Re{(MeO)₂bpy}(CO)₃(MeCN)](PF₆) (sometimes abbreviated as Re(MeO)MeCN)

An acetonitrile solution (60 mL) containing Re{(MeO)₂bpy} (CO)₃Br (601mg, 1.06 mmol) and AgPF₆ (290 mg, 1.15 mmol) was heated to refluxovernight. AgBr thus separated was removed over a Celite layer, and thesolvent was distilled of under reduced pressure. To the resultant,saturated NH₄PF₆ in acetonitrile/water (1:1 v/v) was added, and theacetonitrile was slowly distilled off under reduced pressure to obtain apale yellow solid. This solid was recrystallized fromacetonitrile/diethyl ether/hexane, and a pale yellow solid thus obtainedwas dried under reduced pressure.

Yield: 443 mg (6.59×10⁻¹ mmol), Yield: 62.1%

ESI-MS in MeCN m/z=528 [M-PF₆ ⁻]⁺

ET-IR in MeCN ν(CO)/cm⁻¹: 2038, 1932

¹H-NMR in CD₃CN (298 MHz): δ (ppm)=8.76 (d, J=6.6 Hz, 2H, bpy-6,6′),7.92 (d, 2.7 Hz, 2H, bpy-3,3′), 7.19 (dd, J=2.7, 6.6 Hz, 2H, bpy-5,5′),4.03 (s, 6H, CH₃O), 2.05 (s, 3H, CH₃—CN)

Elemental analysis: Calcd. (%) for C₁₇H₁₅N₃O₅RePF₆: C 30.36; H 2.25; N6.25

Found: C 30.85; H 2.24; N 6.43

Synthesis Example 9

[Re(Br₂bpy)(CO)₃(MeCN)](PF₆) (sometimes abbreviated as Re(Br)MeCN)

An acetonitrile solution (60 mL) containing Re(Br₂bpy) (CO)₃Br (600 mg,9.04×10⁻¹ mmol) and AgPF₆ (242.4 mg, 9.59×10⁻¹ mmol) was heated toreflux overnight. AgBr thus separated was removed over a Celite layer,and the solvent was distilled off under reduced pressure. To theresultant, saturated NH₄PF₆ in acetonitrile/water (1:1 v/v) was added,and the acetonitrile was slowly distilled off under reduced pressure toobtain a pale yellow solid. This solid was recrystallized fromacetonitrile/diethyl ether/hexane, and a reddish brown solid thusobtained was dried under reduced pressure.

Yield: 435 mg (5.65×10⁻¹ mmol), Yield: 62.5%

ESI-MS in MeCN m/z=626 [M-PF₆ ⁻]⁺

FT-IR in MeCN η(CO)/cm⁻¹: 2042, 1941

¹H-NMR in CD₃CN (298 MHz): δ (ppm)=8.81 (d, J=6.0 Hz, 2H, bpy-6,6′),8.72 (d, 2.0 Hz, 2H, bpy-3,3′), 7.92 (dd, J=2.0, 6.0 Hz, 2H, bpy 5,5′),2.06 (s, 3H, CH₃—CN)

Elemental analysis: Calcd. (%) for C₁₅H₉N₃O₃Br₂RePF₆: C 23.39; H 1.18; N5.46

Found: C 23.56; H 1.10; N 5.62

Synthesis Example 10

Re(bpy)(CO)₃(OCOH) (sometimes abbreviated. as Re—OCOH)

An ethanol/water mixed solution (1:1 v/v, 50 mL) containing Re(bpy)(CO)₃Br (301 mg, 5.95×10⁻¹ mmol) and an excessive amount of sodiumformate (4.05 g, 59.6 mmol) was heated to reflux overnight. The ethanolwas slowly distilled off under reduced pressure. Dichloromethane wasadded thereto, and the resultant was extracted with water three times.After distilling off the solvent of the thus obtained organic layerunder reduced pressure, the resultant was recrystallized fromacetone/diethyl ether/hexane, and a yellow solid thus obtained was driedunder reduced pressure.

Yield: 70.4 mg (1.49×10⁻¹ mmol), 25.1%

ESI-MS in MeCN m/z=626 [M-PF₆ ⁻]⁺

FT-IR in CH₂Cl₂ ν(CO)/cm⁻¹: 2022, 1918, 1896

¹H-NMR in CD₃CN (298 MHz): δ (ppm)=9.02 (dd, 2H, J=5.6, 1.6 Hz,bpy-6,6′), 8.40 (dd, 2H, J=8.3, 1.1 Hz, bpy-3,3′), 8.20 (ddd, 23, J=8.3,8.3, 1.6 Hz, bpy-4,4′), 7.61 (ddd, 2H, J=8.3, 5.6, 1.1 Hz, bpy-5,5′),7.81 (s, 1H, HCOO)

Elemental analysis: Calcd. (%) for C₁₄H₉N₂O₅Re: C 35.67; H 1.92; N 5.94

Found: C 35.63; H 1.82; N 6.01

Synthesis Example 11

Re(bpy)(CO)₃(OTf)

A tetrahydrofuran solution (60 mL) containing Re(bpy) (CO)₃Br (1.00 g,1.97 mmol) and AgOTf (553 mg, 2.15 mmol) was heated to reflux overnight.AgBr thus separated was removed over a Celite layer, and the solvent wasdistilled off under reduced pressure. The thus separated solid wasrecrystallized from dichloromethane/dimethyl ether/hexane, and ayellowish brown solid thus obtained was dried under reduced pressure.

Yield: 902 mg (1.56 mmol), Yield: 79.0%

FT-IR ν_(co) [cm⁻¹] in CH₂Cl₂: 2036, 1935, 1915

Synthesis Example 12

[Re{4,4′-(MeO)₂bpy}(CO)₃{P(OC₂H₅)₃}](PF₆)

A tetrahydrofuran solution containing Re[4,4′-(MeO)₂bPY(CO)₃MeCN] (501mg, 7.44×10⁻¹ mmol) and P(OC₂H₅)₃ (ca. 1 mL, 6 nmol) was heated toreflux in a dark place overnight. While blocking light, the solvent andan unreacted portion of P(OC₂H)₃ were distilled off under reducedpressure by using an oil sealed rotary pump. The thus obtained solid wasrecrystallized from dichloromethane/diethyl ether, and the resultant wasseparated and purified by flash column chromatography (elutionsolution:dichloromethane:methanol=100:0 to 100:3 v/v). The thirdfraction thus obtained was further recrystallized fromdichloromethane/dimethyl ethanol, and a pale yellow solid thus obtainedwas dried under reduced pressure. Yield:

413 mg (5.18×10⁻¹ mmol), Yield: 69.6%

ESI-MS in MeCN m/z=653 [M-PF₆ ⁻]⁺

FT-IR in CH₂Cl₂ ν(CO)/cm⁻¹: 2044, 1958, 1925

¹H-NMR in CD₃CN (298 MHz): δ (ppm)=8.59 (d, J=6.4 Hz, 2H, bpy-6,6′),7.94 (d, J=2.6 Hz, 2H, bpy-3,3′), 7.09 (dd, J=2.6, 6.4 Hz, 2H,bpy-5,5′), 4.16 (s, 6H, CH₃O), 3.82 (quip, J=7.1, 7.1, 7.1, 7.1 Hz, 6H,OCH₂CH₃), 1.09 (t, J=7.1, 7.1 Hz, 9H, OCH₂CH₃)

Elemental analysis: Calcd. (%) for C₂₁H₂₇N₂O₈F₆P₂Re: C 31.62; H. 3.41; N3.51

Found: C 31.67; H 3.25; N 3.57

Synthesis Example 13

Re(bpy)(CO)₃(OH)

An acetone/water mixed solution (4:3 v/v, 70 ml,) containing Re(bpy)(CO)₃(OTf) (303 mg, 5.21×10⁻¹ mmol) and potassium hydroxide (1.35 g,24.1×10 mmol) was heated to reflux overnight The acetone was slowlydistilled off under reduced pressure, and a yellow solid thus separatedwas filtered. off and dried under reduced pressure.

Yield: 120 mg (2.71×10⁻¹ mmol), Yield: 51.5%

Synthesis Example 14

Re(bpy)(CO)₃(OCO₂H)

A CO₂ gas was allowed to pass through an acetone solution containingRe(bpy) (CO)₃(OH) (52.0 mg, 1.17×10⁻¹ mmol) for 20 minutes. A yellowsolid thus separated was filtered off and dried under reduced pressure.

Yield: 51.5 mg (1.06×10⁻¹ mmol), Yield: 90.1%

FT-IR in KBr ν(CO)/cm⁻¹: 2022, 1895, 1616, 1602

Synthesis Example 15

[Re(bpy)(CO)₃(DMF)](PF₆)

[Re(bpy)(CO)₃(MeCN)](PF₆) (56.3 mg, 92.0 μmol) was dissolved in DMF-d₇,and the resultant was allowed to stand still in a dark place under Aratmosphere for 12 hours to completely replace a MeCN ligand with DMF.

¹H NMR in DMF-d₇ (500 MHz): δ (ppm)=9.27 (ddd, 2H, J=0.5, 1.0, 5.5 Hz,bpy-6,6′), 8.95 (d, 2H, J=8.0 Hz, bpy-3,3′), 8.54 (ddd, 2H, J=1.0, 8.0,8.0 Hz, bpy-4,4′), 7.97 (ddd, 2H, J=1.0, 5.5, 8.0 Hz, bpy-5,5′)

¹³C NMR in DMF-d₇ (126 MHz): δ (ppm)=196.9, 193.2, 156.5, 154.9, 142.0,128.9, 125.2

FT-IR in DMF ν(CO)/cm⁻¹: 2029, 1922, 1913

Synthesis Example 16

Re(bpy) (CO)₃{O—CO—OCH₂CH₂N(CH₂CH₂OH)₂}

To a DMF solution (2 mL) containing [Re(bpy)(CO)₃(DMF)]⁺,triethanolamine (TEOA, 200 μL) was added. The resultant was allowed tostand still for 12 hours to partially replace a DMF ligand with TEOA,and thus, the resultant was changed to an equilibrium mixture of[Re(bpy)(CO)₃(DMF)]⁺ and Re(bpy)(CO)₃(TEOA).

Through the resultant solution, CO₂ was allowed to pass for 30 minutes.At this point, Re(bpy)(CO)₃OCO₂H was precipitated and hence was filteredoff, and the resultant filtrate was used as a sample solution for an NMRspectrum.

¹H NMR in DMF-d₇-TEOA (10:1 v/v) (500 MHz): δ (ppm)=9.19 (ddd, 2H,J=0.5, 1.0, 5.5 Hz, bpy-6,6′), 8.82 (d, 2H, J=8.0 Hz, bpy-3,3′), 8.42(ddd, 2H, J=1.0, 8.0, 8.0 Hz, bpy-4,4′), 7.87 (ddd, 2H, J=1.0, 5.5, 8.0Hz, bpy-5,5′)

¹³C NMR in DMF-d₇-TEOA (10:1 v/v) (126 MHz): δ (ppm)=198.4, 194.4, 158.4(C═O), 156.0, 153.8, 140.9, 128.0, 124.4

FT-IR in DMF-TEOA (5:1 v/v) ν(CO)/cm⁻¹: 2020, 1915, 1892

ESI-MS in MeCN m/z=620 [M+H⁺-PF₆ ⁻]⁺, 642 [M+Na⁺-PF₆ ⁻]⁺

Synthesis Example 17

Mn(bpy)(CO)₃Br

A diethyl ether solution (400 mL) containing 2,2′-bipyridine (bpy) (0.57g, 3.65 mmol) and Mn(CO)₃Br (1.0 g, 3.65 mmol) was heated to reflux for3 hours. An orange powder thus separated. was filtered off, washed withdiethyl ether, and dried under reduced pressure.

Yield: 1.26 g (92.6%)

¹H NMR (400 MHz, aceton-d₆, ppm) δ=9.30 (d, 2H, J=4.8 Hz; 2H; H6,6′),8.58 (d, 2H, J=8.2 Hz, H3,3′), 8.23 (td, 2H, J=5.9 Hz), 7.75 (t, J=5.9Hz, 2H; H5,5′).

FT-IR (CH₂Cl₂): ν(CO)/cm⁻¹, 2028, 1938, 1922.

Synthesis Example 18

[Mn(bpy)(CO)₃CH₃CN]PF₆

An acetonitrile solution (350 mL) containing AgPF₆ (0.69 g, 3.65 mmol)and Mn(bpy)(CO)₃Br (1.00 g, 2.71 mmol) was heated to 40° C. for 1 hour.The thus obtained mixture was filtered through Celite. The resultantfiltrate was evaporated to dryness, ant the thus obtained solid waswashed with diethyl ether and dried under reduced pressure.

Yield: 1.23 g (96.0%)

¹H NMR (400 MHz, CD₃Cl₃, ppm) δ=9.04 (d, 2H, J=3.6 Hz; 2H; H6,6′), 8.41(d, 2H, J=7.6 Hz, H3,3′), 8.22 (td, 2H, J=5.9 Hz, H4,4′), 7.66 (t, 2H,J=5.9 Hz, H5,5′), 2.10 (s, 3H, CH₃)

FT-IR (CH₃CN): ν(CO)/cm⁻¹, 2028, 1938, 1923.

Elemental Anal. Calcd (%) for C₁₃H₈BrMnN₂O₃: C, 37.44: H, 2.30; N, 8.73.

Found: C, 37.56: H, 2.21; N, 8.83.

Synthesis Example 19

Mn(MeObpy)(CO)₃Br

Mn(MeOpby)(CO)₃Br was obtained in the same manner as the synthesis ofMn(bpy)(CO)₃Br of Synthesis Example 18.

Yield: 96.3%

¹H NMR (400 MHz, aceton-d₆, ppm) δ=9.02 (d, 2H, J=6.4 Hz; H6,6′), 8.13(d, 2H, J=2.0 Hz, H3,3′), 7.30 (dd, 2H, J=6.4, 2.0 Hz, H5,5′), 4.12 (s,6H, OCH₃)

FT-IR (CH₂Cl₂): ν(CO)/cm⁻¹ 2026, 1930, 1918.

Synthesis Example 20

[Mn(MeObpy)(CO)₃CH₃CN]PF₆

[Mn(MeObpy)(CO)₃CH₃CN]PF₆ was obtained in the same manner as thesynthesis of [Mn(bpy)(CO)₃CH₃CN]PF₆ of Synthesis Example 18

Yield: 75.1%

¹H NMR (400 MHz, CD₃Cl₃, ppm) δ=8.72 (d, 2H, J=6.4 Hz; 2H; H6,6′) , 7.81(d, 2H, J=2.6Ha, H3,3′), 7.82 (td, 2H, J=2.6, 6.4 Hz, H4,4′), 4.11 (s,6H, OCH₃) , 2.16 (s, 3H, NCCH₃)

FT-IR (CH₃CN): ν(CO)/cm⁻¹, 2047, 1953.

Synthesis Example 21

Mn(Brbpy)(CO)₃Br

Mn(Brbpy)(CO)₃Br was obtained in the same manner as the synthesis ofMn(bpy)(CO)₃Br of Synthesis Example 17.

Yield: 0.64 g (94.5%)

¹H NMR (400 MHz, aceton-d₆, ppm) δ=9.17 (d, 2H, J=6.0 Hz, 2H, H6,6′),8.94 (d, 2H, J=1.8 Hz, H3,3′), 8.01 (dd, 2H, J=5.6, 1.8 Hz, H5,5′)

FT-IR (CH₂Cl₂): ν(CO)/cm⁻¹, 2030, 1938.

Synthesis Example 22

[Mn(Brbpy)(CO)₃CH₃CN]PF₆

[Mn(Brbpy)(CO)₃CH₃CN]PF₆ was obtained in the same manner as thesynthesis of [Mn(bpy)(CO)₃CH₃CN]PF₆ of Synthesis Example 18.

Yield: 1.23 g (96.6%)

¹H NMR (400 MHz, CDCl₃, ppm) δ=8.83 (d, 2H, J=5.8 Hz; 2H; H6,6′), 8.42(d, 2H, J=2.2 Hz, H3,3′), 7.82 (td, 2H, J=2.2, 5.8 Hz, H4,4′), 2.16 (s,3H, CH₃, H)

FT-IR (CH₃CN): ν(CO)/cm⁻¹, 2051, 1963.

Elemental Anal. Calcd (%) for C₁₅H₉Br₂MnN₃O₃: C, 28.20; H, 1.42; N,6.58.

Found: C, 28.50; H, 1.28; N, 6.69.

Synthesis Example 23 (CO₂ Addition Reaction to Metal Complex of Formula(1))

(1) Generation of Re—CO₂-TEOA in DMF-TEOA Mixed Solution

CO₂ was allowed to pass through a solution containing a rhenium complexto attempt to observe a rhenium complex to which CO₂ had been added.

Re—MeCN was dissolved in DMF to a concentration of 5.30 mM, and theresultant was allowed to stand still in a dark place under Aratmosphere. Thereafter, TEOA was added thereto, and the resultant wasallowed to stand still again in a dark place under Ar atmosphere. Then,CO₂ was allowed to pass through the thus obtained DMF-TEOA mixedsolution for 30 minutes. IR spectrum change of the resultant solution isillustrated in FIG. 3. Besides, change in color of the solution wasobserved. Furthermore, Ar was allowed to pass for 2 hours through thesolution through which CO₂ had passed, so as to remove dissolved CO₂from the solution. IR spectrum change thus caused is illustrated in FIG.4.

When CO₂ was allowed to pass through this equilibrium mixture, all peaksprecedently observed disappeared, and new peaks were observed at 2020cm⁻¹, 1915 cm⁻¹ and 1892 cm⁻¹ (FIG. 3). It can be determined based onthe shapes of these IR peaks that a newly produced complex retains atricarbonyl structure. Besides, when CO₂ was allowed to pass through,the color of the solution was changed from reddish brown to yellow,which suggests that some sort of reaction was caused between Re-DMF orRe-TEOA and CO₂. Even if CO₂ was allowed to pass through a DMF solutioncontaining Re-DMF, the IR spectrum was not changed. Accordingly, it ispresumed that the reaction is caused between Re-TEOA and CO₂. Besides,the peak of the newly produced complex (Re—CO₂-TEOA) is positioned on alower frequency side of the peak of Re-DMF but on a higher frequencyside of the peak of Re-TEOA. This relationship in the wavelength revealsthat the electron-donating property of a ligand of Re—CO₂-TEOA isstronger than that of a DMF ligand but weaker than that of a TEOAligand.

When Ar was allowed to pass through, the peaks of Re—CO₂-TEOA werelargely reduced, and mainly the peaks of Re-TEOA were recovered. After 2hours, the peaks of Re—CO₂-TEOA completely disappeared (FIG. 4) , and aconcentration ratio between Re-DMF and Re-TEOA became substantially thesame as that in the equilibrium state obtained before allowing CO₂ topass therethrough. Accordingly, it was found that the generationreaction of Re—CO₂-TEOA is reversible and that the equilibrium dependson the CO₂ concentration in the solution.

(2) Determination of Structure of Re—CO₂-TEOA

An attempt was made to specify the structure of Re—CO₂-TEOA from the MSspectrum and the NMR spectrum thereof.

FIG. 5 illustrates an PSI-MS spectrum of a DMF-TEOA mixed solution (5:1v/v) containing Re—CO₂-TEOA. Peaks were mainly observed at not onlyRe—MeCN (m/z=468), Re-DMF (m/z=500) and Re-TEOA (m/z=576) but alsom/z=620.

The peak at m/z=620 corresponds to a monovalent complex (m/z=619.62)resulting from addition of CO₂ (exact mass=44.01) and TEOA (exactmass=149.19) to Re(bpy)(CO)₃ (exact mass 426.42).

It was concluded, on the basis of ¹H-NMR and ¹³C-NMR, thatRe(bpy)(CO)₃{O—CO—OCH₂CH₂N(CH₂CH₂OH)₂} represented by the followingformula had been generated:

(3) Uptake of CO₂ from Air by Re-TEOA

With general air allowed to pass through instead of 100% CO₂ gas, it waschecked whether or not CO₂ in the ambient air was taken in by Re-TEOA.Besides, with an electron-withdrawing or electron-donating substituentintroduced into the 4,4′-position of the bpy ligand, the relationshipbetween the electron density of the central metal and CO₂ uptake abilitywas also checked.

Re—MeCN, Re(Me)MeCN, Re(MeO)MeCN and Re(Br)MeCN respectively containinga bpy ligand in which hydrogen, a methyl group, a methoxy group or abromo group was substituted in the 4,4′-position were synthesized. Eachof these complexes was dissolved in DMF, the resultant was allowed tostand still in a dark place under Ar atmosphere overnight, and then,TEOA was added thereto, the resultant was allowed to stand still in adark place under Ar atmosphere for 2 hours, and thus, a DMF-TEOA mixedsolution (5:1 v/v) containing the complex in which DMF or TEOA wascoordinated was prepared. Change in the IR spectrum caused by allowingthe ambient air to pass through each of such solutions for 1 to 2 hoursby using a diaphragm pump was observed. Besides, curve fitting analysiswas performed in a range of 2060 to 1980 cm⁻¹ so as to separate peaks ofthe respective complexes. FIG. 6 illustrates both the IR spectra andcurve fitting curves of the respective solutions.

As a result, it was found that in all of the rhenium complexescontaining any one of the bpy ligands, a part of each complex havingTEOA coordinated therein took in CO₂ contained in the passing air.Besides, 10 to 30% of all the rhenium complexes took CO₂ from theambient air. These results reveal that a rhenium complex works as a goodCO₂ absorbent. Furthermore, since a gas containing water vapor wasallowed to pass, it was predicted that water was unavoidably supplied tothe solution and hence generation of Re—OH and Re—OCO₂H might compete.After the ambient air was allowed to pass, however, no yellow solid wasseparated, and hence, it is presumed that none of these complexes weregenerated or their amounts was ignorably small.

It was found that CO₂ uptake efficiency is largely varied depending on asubstituent contained in the bpy ligand and that the equilibrium thereofaccords with Hammett rule. A positive correlation (ρ=0.8>0) was found ina Hammett plot, and it was found that the CO₂ uptake by Re-TEOA tends tomore easily occur as the charge density of rhenium is smaller.

Besides, 1% CO₂ or 10% CO₂ was allowed to pass instead of the 100% CO₂gas so as to check whether or not CO₂ could be taken in by Re-TEOA, andas a result, it was found that CO₂ was taken in at high frequency andthat a similar compound, Re(bpy)(CO)₃(OCOCH₂CH₂)N(CH₂CH₂OH)₂ had beengenerated.

(4) Each of Mn(bpy)(CO)₃(MeCN), Mn(MeObpy)(CO)₃(MeCN) and.Mn(Brbpy)(CO)₃(MeCN) was dissolved in DMF containing triethanolamine(TEOA), and CO₂ was blown thereinto, so as to examine, through FT-IRmeasurement, whether or not CO₂ was taken in in the same manner as inusing the Re complexes (FIG. 13).

As a result, it was found that the following reactions had occurred:

(5) Besides, it was examined whether or not CO₂ was taken in even if CO₂at a low concentration was used. The uptake of CO₂ was examined byblowing the ambient air instead of CO₂. As a result, uptake of 21.7%,14.7% and 37.1% of CO₂ was observed respectively in usingMn(bpy)(CO)₃{OCH₂CH₂N(CH₂CH₂OH)₂}, Mn(MeObpy)(CO)₃{OCH₂CH₂N(CH₂CH₂OH)₂},and Mn(Brbpy)(CO)₃{OCH₂CH₂N(CH₂CH₂OH)₂}.

(6) Besides, 1% CO₂, 2% CO₂, 5% CO₂ or 10% CO₂ was allowed to passthrough instead of 100% CO₂ gas, and the CO₂ uptake byMn(bpy)(CO)₃(OCH₂CH₂NH(CH₂CH₂OH)₂), Mn(MeObpy)(CO)₃(OCH₂CH₂N(CH₂CH₂OH)₂)or Mn(Brbpy)(CO)₃(OCH₂CH₂N(CH₂CH₂OH)₂) was examined. As a result, it wasfound, as shown in Table 1, that CO₂ was efficiently taken in.

TABLE 1

Proportion of CO₂ adduct complex generated /% Proportion of CO₂ gas/% 12 5 10 X = MeO — 55.7 62.4 71.0 X = H 54.1 58.2 65.3 74.3 X = Br — 75.579.6 88.9

(7) The CO₂ uptake by a Mn complex in which another compound wascoordinated instead of triethanolamine was examined.

A Mn complex (Mn(bpy)(CO)₃(CH₃CN)) was dissolved in DMF-TEA(triethylamine), and CO₂ was blown into the resultant. As a result,Mn(bpy)(CO)₃(OEt) was generated, and a CO₂ adduct of this, that is,Mn(bpy)(CO)₃OC(O)OEt, was found to be generated.

(8) The CO₂ uptake by a Mn complex in which diethanolamine (DEOA) wascoordinated instead of triethanolamine was examined.

When a Mn complex was dissolved in a DMF-DEOA mixed solution and CO₂ wasblown into this solution, a reaction with DEOA present in the solutionwas caused to generate carbamic acid, and it was found that a carbamatecomplex Mn(bpy)(CO)₃(OCON(CH₂CH₂OH)₂) was generated through coordinationof the carbamic acid in the complex. FIG. 14 illustrates IR spectrumchange of the Min complex in DMF-DEOA under CO₂ atmosphere at variousconcentrations.

(9) The CO₂ uptake by a Mn complex in which diethylamine (DEA) wascoordinated instead of triethanolamine was examined.

As a result, Mn(bpy)(CO)₃(DEA) was generated from Mn(bpy)(CO)₃(CH₃CN),and a CO₂ adduct of this, that is, Mn(bpy)(CO)₃(OCON(Et)₂), wasconfirmed to be generated.

(10) The CO₂ uptake by a Re complex in which another compound wascoordinated instead of triethanolamine was examined.

DEOA (diethanolamine) was caused to work on a Re complex(Re(bpy)(CO)₃(CH₃CN) in DMF, and CO₂ was blown into the resultant. As aresult, Re(bpy)(CO)₃(NH(CH₂CH₂OH)₂) was generated, and a CO₂ adduct ofthis, that is, Re(bpy)(CO)₃(OCON(CH₂CH₂OH)₂), was found to be generated.

(11) The CO₂ uptake by a Re complex (Re(bpy)(CO)₃(CH₃CN) in whichdiethylamine (DEA) was coordinated) was examined.

As a result, Re(bpy)(CO)₃(DEA) was generated from (Re(bpy)(CO)₃(CH₃CN),and a CO₂ adduct of this, that is, Re(bpy)(CO)₃(OCON(Et)₂), wasconfirmed to be generated.

Test Example 1 (Generation of CO through Electrochemical Reduction UsingRe Complex)

An H-type electrochemical cell including an ion exchange membrane(Nafiion-H) disposed inside as schematically illustrated in FIGS. 1 and2 was produced. On a working electrode side, 84 of a DMF-TEOA (in avolume ratio of 5:1) solution containing 0.5 mMRe(4,4′-(Me)₂(bpy)(CO)₃OCOOCH₂CH₂N(CH₂CH₂OH) and 0.1 M Et₄NBF₄ wasadded. On the other hand, on a counter electrode side, 84 mL of aDMF-TEOA (in a volume ratio of 5:1) solution containing 0.1 M Et₄NBF₄was added. Netted glassy carbon (glassy carbon) was used as a workingelectrode, a platinum cross mesh electrode was used as a counterelectrode, and a silver/silver nitrate electrode was used as a referenceelectrode. FIG. 9 illustrates results of cyclic voltammetry measurementperformed for setting an application voltage in this Test Example 1.Based on FIG. 9, a conspicuous current response was found in thevicinity of an application voltage of −2.0 V, and it is determined thata catalytic reduction reaction of CO₂ occurred at this voltage.

On the working electrode side, 10% CO₂, 50% CO₂ or 100% CO₂ (a componentexcluding CO₂ being Ar) was bubbled, and a voltage of −2.1 V (usingAg/AgNO₃ as the reference electrode) was applied. As a result, it wasfound, as illustrated in FIG. 7, that CO₂ was selectively reduced to COeven if the CO₂ concentration was 10%. The faradaic efficiency of the COgeneration was substantially 100%.

Test Example 2

An electrochemical cell similar to that of Test Example 1 except thatthe solvent DMF-TEOA used on the working electrode side was changed toDMT was produced. While bubbling 10% CO₂ gas, 7 of TEOA was added on theworking electrode side, and change in a current at which CO wasgenerated was observed.

As a result, as illustrated in FIG. 8, the current value was increasedby about 6 times from 0.76 mA to 4.85 mA, and it was understood that CO₂reducing ability could be further improved by causing CO₂ additionfunction (by shifting the equilibrium relationship toward side ofgeneration of a CO₂ adduct) through addition of TEOA.

Test Example 3(Generation of Formic Acid Through ElectrochemicalReduction Using Mn Complex and TEOA)

An H-type electrochemical cell including an ion exchange membrane(Nafiion-H) disposed inside was produced. On a working electrode side,84 mL of a DMF-TEOA solution (TEOA: 1.26 M) containing 0.5 mMMn(bpy)(CO)₃OCOOCH₂CH₂N(CH₂CH₂OH) and 0.1 M TEABF₄ was added. On theother hand, on a counter electrode side, 84 mL of a DMF-TEOA solution(TEOA: 1.26 M) containing 0.1 M TEABF₄ was added. Netted glassy carbon(glassy carbon) was used as a working electrode, a platinum cross meshelectrode was used as a counter electrode, and a silver/silver nitrateelectrode was used as a reference electrode. FIG. 10 illustrates resultsof the cyclic voltammetry measurement performed for setting anapplication voltage in this Test Example 3. Based on FIG. 10, aconspicuous current response was found in the vicinity of an applicationvoltage of −2.0 V, and it is determined that a catalytic reductionreaction. of CO₂ occurred at this voltage.

On the working electrode side, 10% CO₂ or 100% CO₂ (a componentexcluding CO₂ Ar) was bubbled, and a voltage of 2.0 V (using Aq/AgNO₃ asthe reference electrode) was applied. As a result, it was found, asillustrated in FIGS. 12 and 13, that formic acid was selectivelygenerated even if the CO₂ concentration was 10%.

Test Example 4 (Generation of Formic Acid Through ElectrochemicalReduction Using Mn Complex and DEOA)

An H-type electrochemical cell including an ion exchange membrane(Nafiion-H) disposed inside was produced. On a working electrode side,95 mL of a DMF solution containing 0.5 mM Mn(bpy)(CO)₃(OCONCH₂CH₂NR₂)(R═CH₂CH₂OH), 0.1 M Et₄NBF₄, 0.62 M DEOA and 0.62 M tripropylamine wasadded. On a counter electrode side, 95 mL of a DMF solution containing0.1 M Et₄NBF₄, 0.62 M DEOA and 0.62 M tripropylamine was added. Nettedglassy carbon was used as a working electrode, a platinum mesh electrodewas used as a counter electrode, and a silver/silver nitrate electrodewas used as a reference electrode. FIG. 15 illustrates results of thecyclic voltammetry measurement performed for setting an applicationvoltage in this Test Example 4. Based on FIG. 15, a conspicuous currentresponse was found in the vicinity of an application voltage of −1.85 V,and it is determined that a catalytic reduction reaction of CO₂ occurredat this voltage.

While bubbling CO₂ gas on the working electrode side, a voltage of −1.85V (using Ag/AgNO₃ as the reference electrode) was applied. As a result,it was found that CO₂ was highly selectively reduced to HCOOH.

1: A method for producing carbon monoxide by electrochemically reducingcarbon dioxide, the method comprising: (a) reacting carbon dioxide witha metal complex represented by formula (1):

wherein X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³, Y represents CO, OR¹,SR¹, NR²R³R⁴ or PX¹X²X³, ring A and ring B are identical or differentand represent a nitrogen atom-containing heterocycle optionally having asubstituent, R¹ represents a hydrocarbon group optionally having asubstituent, one, two or three of R², R³ and R⁴ are identical ordifferent and represent a hydrocarbon group optionally having asubstituent, the rest representing a hydrogen atom, and one, two orthree of X¹, X² and X³ are identical or different and represent ahydrocarbon group optionally having a substituent or a hydrocarbon oxygroup optionally having a substituent, the rest representing a hydrogenatom or a hydroxy group; and (b) applying a voltage to a reactionproduct of the carbon dioxide and the metal complex represented byformula (1). 2: The production method according to claim 1, wherein thesteps (a) and (b) are performed within an electrochemical cell includinga working electrode and a counter electrode, and the method comprises:(a1) introducing carbon dioxide into a solution comprising the metalcomplex held in the electrochemical cell; and (b1) applying a negativevoltage and a positive voltage respectively to the working electrode andthe counter electrode of the electrochemical cell. 3: The productionmethod according to claim 2, wherein the carbon dioxide is introduced byintroducing a carbon dioxide-containing gas into the solution containingthe metal complex. 4: The production method according to claim 1,wherein the carbon dioxide to be reacted is a gas containing 0.03 to100% of carbon dioxide. 5: The production method according to claim 1,wherein the nitrogen atom-containing heterocycle is a heterocycle havinga 2,2′-bipyridine structure optionally having a substituent. 6: Theproduction method according to claim 1, wherein each hydrocarbon groupoptionally having a substituent represented by R¹, R², R³, R⁴, X¹, X²and X³ is one selected from the group consisting of an alkyl group, analkenyl group, a cycloalkyl group, a cycloalkenyl group and an aromatichydrocarbon group, each of which optionally has one to threesubstituents selected from the group consisting of a primary, secondaryor tertiary amino group, a hydroxy group, as alkoxy group, as aryloxygroup, a halogen atom, a nitro group, a cyano group, a formyl group, analkanoyl group and an arylcarbonyl group. 7: A method for producingcarbon monoxide from carbon dioxide, wherein the carbon monoxideobtained by the production method according to claim 1 is used as areducing agent. 8: A method for producing a hydrocarbon-based compound,wherein the carbon monoxide obtained by the production method accordingto claim 1 is used as a raw material. 9: A catalyst forelectrochemically reducing carbon dioxide to carbon monoxide, thecatalyst represented by formula (1):

wherein X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³, Y represents CO, OR¹,SR¹, NR²R³R⁴ or PX¹X²X³, ring A and ring B are identical or differentand represent a nitrogen atom-containing heterocycle optionally having asubstituent, R¹ represents a hydrocarbon group optionally having asubstituent, one, two or three of R², R³ and R⁴ are identical ordifferent and represent a hydrocarbon group optionally having asubstituent, the rest representing a hydrogen atom, and one, two orthree of X¹, X² and X³ are identical or different and represent ahydrocarbon group optionally having a substituent or a hydrocarbon oxygroup optionally having a substituent, the rest representing a hydrogenatom or a hydroxy group. 10: The catalyst according to claim 9, whereinthe nitrogen atom-containing heterocycle is a heterocycle having a2,2′-bipyridine structure optionally having a substituent. 11: Thecatalyst according to claim 9, wherein each hydrocarbon group optionallyhaving a substituent represented by R¹, R², R³, R⁴, X¹, X² and X³ is oneselected from the group consisting of an alkyl group, an alkenyl group,a cycloalkyl group, a cycloalkenyl group and an aromatic hydrocarbongroup, each of which optionally has one to three substituents selectedfrom the group consisting of a primary, secondary or tertiary aminogroup, a hydroxy group, as alkoxy group, as aryloxy group, a halogenatom, a nitro group, a cyano group, a formyl group, an alkanoyl groupand an arylcarbonyl group. 12: A method for producing formic acid byelectrochemically reducing carbon dioxide, the method comprising: (a)reacting carbon dioxide with a metal complex represented by formula (2):

wherein M₁ represents manganese, ruthenium or iron, X represents OR¹,SR¹, NR²R³R⁴ or PX¹X²X³, Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,ring A and ring B are identical or different and represent anitrogen-containing heterocycle optionally having a substituent, R¹represents a hydrocarbon group optionally having a substituent, one, twoor three of R², R³ and R⁴ are identical or different and represent ahydrocarbon group optionally having a substituent, the rest representinga hydrogen atom, and one, two or three of X¹, X² and X³ are identical ordifferent and represent a hydrocarbon group optionally having asubstituent or a hydrocarbon oxy group optionally having a substituent,the rest representing a hydrogen atom or a hydroxy group; and (b)applying a voltage to a reaction product of the carbon dioxide and themetal complex represented by formula (2). 13: The production methodaccording to claim 12, wherein the steps (a) and (b) are performedwithin an electrochemical cell including a working electrode and acounter electrode, and the method comprises: (a1) introducing carbondioxide into a solution containing the metal complex held in theelectrochemical cell; and (b1) applying a negative voltage and apositive voltage respectively to the working electrode and the counterelectrode of the electrochemical cell. 14: The production methodaccording to claim 13, wherein the carbon dioxide is introduced byintroducing a carbon dioxide-containing gas into the solution containingthe metal complex. 15: The production method according to claim 12,wherein the carbon dioxide to be reacted is a gas containing 0.03 to100% of carbon dioxide. 16: The production method according to claim 12,wherein the nitrogen atom-containing heterocycle is a heterocycle havinga 2,2′-bipyridine structure optionally having a substituent. 17: Theproduction method according to claim 12, wherein each hydrocarbon groupoptionally having a substituent represented by R¹, R², R³, R⁴, X¹, X²and X³ is one selected from the group consisting of an alkyl group, analkenyl group, a cycloalkyl group, a cycloalkenyl group and an aromatichydrocarbon group, each of which optionally has one to threesubstituents selected from the group consisting of a primary, secondaryor tertiary amino group, a hydroxy group, an alkoxy group, an aryloxygroup, a halogen atom, a nitro group, a cyano group, a formyl group, analkanoyl group and an arylcarbonyl group. 18 : A catalyst forelectrochemically reducing carbon dioxide to formic acid, the catalystrepresented by formula (2):

wherein M₁ represents manganese, ruthenium or iron, X represents OR¹,SR¹, NR²R³R⁴ or PX¹X²X³, Y represents CO, OR¹, SR¹, NR²R³R⁴ or PX¹X²X³,ring A and ring B are identical or different and represent a nitrogenatom-containing heterocycle optionally having a substituent, R¹represents a hydrocarbon group optionally having a substituent; one, twoor three of R², R³ and R⁴ are identical or different and represent ahydrocarbon group optionally having a substituent, the rest representinga hydrogen atom, and one, two or three of X¹, X² and X³ are identical ordifferent and represent a hydrocarbon group optionally having asubstituent or a hydrocarbon oxy group optionally having a substituent,the rest representing a hydrogen atom or a hydroxy group. 19: Thecatalyst according to claim 18, wherein the nitrogen atom-containingheterocycle is a heterocycle having a 2,2′-bipyridine structureoptionally having a substituent. 20: The catalyst according to claim 18,wherein each hydrocarbon group optionally having a substituentrepresented by R¹, R², R³, R⁴, X¹, X² and X³ is one selected from thegroup consisting of an alkyl group, an alkenyl group, a cycloalkylgroup, a cycloalkenyl group and an aromatic hydrocarbon group, each ofwhich optionally has one to three substituents selected from the groupconsisting of a primary, secondary or tertiary amino group, a hydroxygroup, an alkoxy group, an aryloxy group, a halogen atom, a nitro group,a cyano group, a formyl group, an alkanoyl group and an arylcarbonylgroup. 21: A metal complex represented by formula (2a):

wherein M₁ represents manganese, ruthenium or iron, X representsO(CH₂)_(n)NR⁵R⁶, NR⁵R⁶ or PX¹X²X³, Y represents CO, C(CH₂)_(n)NR⁵R⁶,NR⁵R⁶ or PX¹X²X³, ring A and ring B are identical or different andrepresent a nitrogen atom-containing heterocycle optionally having asubstituent, one, two or three of X¹, X² and X³ are identical ordifferent and represent a hydrocarbon group optionally having asubstituent or a hydrocarbon oxy group optionally having a substituent,the rest representing a hydrogen atom or a hydroxy group, R⁵ and R⁶ areidentical or different and represent an alkyl group, a hydroxyalkylgroup or a hydrogen atom, and n represents a number of 2 to
 8. 22: Themetal complex according to claim 21, wherein the nitrogenatom-containing heterocycle is a heterocycle having a 2,2′-bipyridinestructure optionally having a substituent. 23: The metal complexaccording to claim 21, wherein each hydrocarbon group optionally havinga substituent represented by X¹, X² and X³ is one selected from thegroup consisting of an alkyl group, an alkenyl group, a cycloalkylgroup, a cycloalkenyl group and an aromatic hydrocarbon group, each ofwhich optionally has one to three substituents selected from the groupconsisting of a primary, secondary or tertiary amino group, a hydroxygroup, an alkoxy group, an aryloxy group, a halogen atom, a nitro group,a cyano group, a formyl group, an alkanoyl group and an arylcarbonylgroup. 24: A carbon monoxide production system for producing carbonmonoxide by electrochemically reducing carbon dioxide, the systemcomprising: an electrochemical cell part equipped with a solutioncontaining a metal complex, a working electrode and a counter electrode;an injection part through which carbon dioxide is injected into thesolution containing the metal complex held in the electrochemical cellpart; a voltage source capable of applying a positive or negativevoltage between the working electrode and the counter electrode of theelectrochemical cell part; and a discharge part discharging carbonmonoxide generated within the solution containing the metal complex,wherein the carbon monoxide is generated by applying a positive ornegative voltage to a reaction product of the metal complex generated bythe solution containing the metal complex and the carbon dioxide. 25:The carbon monoxide production system according to claim 24, wherein:the metal complex is represented by formula (1):

X represents OR¹, SR¹, NR²R³R⁴ or PX¹X²X³, Y represents CO, OR¹, SR¹,NR²R³R⁴ or PX¹X²X³, ring A and ring B are identical or different andrepresent a nitrogen atom-containing heterocycle optionally having asubstituent, R¹ represents a hydrocarbon group optionally having asubstituent, one, two or three of R², R³ and R⁴ are identical ordifferent and represent a hydrocarbon group optionally having asubstituent, the rest representing a hydrogen atom, and one, two orthree of X¹, X² and X³ are identical or different and represent ahydrocarbon group optionally having a substituent or a hydrocarbon oxygroup optionally having a substituent, the rest representing a hydrogenatom or a hydroxy group. 26: The carbon monoxide production systemaccording to claim 24, wherein the carbon dioxide is fed withoutconcentration in a feed part feeding the carbon dioxide. 27: The carbonmonoxide production system according to claim 24, further comprising: acarbon monoxide detection part detecting a concentration of the carbonmonoxide discharged from the solution containing the metal complex. 28:The carbon monoxide production system according to claim 27, wherein thecarbon monoxide detection part is a gas chromatography. 29: The carbonmonoxide production system according to claim 24, wherein the nitrogenatom-containing heterocycle is a heterocycle having a 2,2′-bipyridinestructure optionally having a substituent. 30: The carbon monoxideproduction system according to claim 24, wherein each hydrocarbon groupoptionally having a substituent represented by R¹, R², R³, R⁴, X¹, X²and X³ is one selected from the group consisting of an alkyl group, analkenyl group, a cycloalkyl group, a cycloalkenyl group and an aromatichydrocarbon group, each of which optionally has one to threesubstituents selected from the group consisting of a primary, secondaryor tertiary amino group, a hydroxy group, an alkoxy group, an aryloxygroup, a halogen atom, a nitro group, a cyano group, a formyl group, analkanoyl group and an arylcarbonyl group.